How to get notified when a zeroing weak reference becomes nil on Objective-C under ARC?

Question

Is there a mechanism which would allow an object to know that a zeroing weak reference turned nil?

For example I have a property

@property (nonatomic, weak) MyClass *theObject; 

when theObject deallocates and the property turns nil I want to get notified. But how? Does the zeroing weak reference system use the setter to set the property to nil when the object goes away?

Answer 1

The runtime just sets the weak ivar _theObect to nil, a custom setter is not called.

What you could do (if you really need the notification):

• define a local "watcher" class and implement dealloc in that class,
• create a watcher object and set it as "associated object" of _theObject.

When _theObject is deallocated, the associated object is released and deallocated (if there are no other strong refereces to it). Therefore its dealloc method is called. This is your "notification".

(I'm writing this on the phone and can fill in the details later if necessary.)